1. Field of the Invention
The present invention relates to an image forming apparatus based on electrophotography such as a copier, a printer or a facsimile and, more specifically, to an image forming apparatus provided with a development member such as a developing roller.
2. Description of the Related Art
In an image forming apparatus based on electrophotography, an electrostatic latent image on a photoreceptor as an image carrier is developed by a developing device with toner, and a toner image is formed. The toner image on the photoreceptor is transferred, for example, to a sheet of recording paper. In such a process of transfer in the image forming apparatus, generally, electrostatic transfer is utilized.
When a toner image is to be transferred to a sheet of paper as a receiver, a voltage is applied by a transfer roller or the like from the rear surface of the sheet arranged to face the photoreceptor, whereby an electric field is formed between the photoreceptor and the recording paper and by the electric field, the toner image is attracted in electrostatic manner to the recording paper.
Thereafter, the transferred toner image is pressed by a fixing device and fixed on the recording paper.
Dry and wet developing devices have been conventionally known as developing devices for the image forming apparatuses. In both types of devices, developer is fed to a developing roller, which is an elastic member, by means of a conveyer roller, and an electrostatic latent image on the photoreceptor is developed by the developing roller. In such a developing device, while an image is not formed, that is, in a period of non-image-formation, the developing roller, which is an elastic member, is in a stationary state.
When a surface of an elastic member (developing roller) is kept partially in pressure contact with another member (for example, a carrying roller), compressive strain generates at the portion of pressure contact on the surface of the elastic member. At the portion where the compressive strain generates, the force of pressure contact between the members decreases and, therefore, when the elastic member is rotated again for image formation, liquid developer cannot be passed uniformly, resulting in uneven density in a rotation period of the elastic member.
In the following, the mechanism how the generation of strain causes uneven density will be described, taking a wet developing device as an example.
When a member on the side of passing the liquid developer and a member on the side of receiving the developer are rotating in the same direction at a portion where these members are opposite to each other, generally, the liquid developer passes through the nip between the members and distributed to respective members at a ratio in accordance with the ratio of speed of the members.
If there is a strain on the surface of the elastic member, however, larger amount of liquid developer would be carried at the portion dented because of the strain.
Therefore, if there is a strain on the passing side member and large amount of developer is carried thereon, the amount fed to the receiving side member opposite thereto also increases.
Next, when a member on the side of passing the liquid developer and a member on the side of receiving the developer are rotating in the opposite directions at the portion where these members are opposite to each other, the liquid developer on the passing side member does not pass through the nip portion but fed to the receiving side member before reaching the nip. If there is a strain on the surface of passing side member, however, the force of pressure contact reduces at this portion and, therefore, the liquid developer tends to easily go through the nip. Where the developer slips through the nip, the amount of developer fed to the receiving side member decreases.
Because of the above-described mechanism, when there is a strain on the elastic member, the amount of liquid developer carried thereon increases or decreases, resulting in uneven density in the rotation period of the strained elastic member.
Such a problem of uneven density is also experienced in the dry developing device.
Japanese Laid-Open Patent Publication No. 02-248967 discloses, in a recording device in which development is done by a developing roller and a photoreceptor drum brought into contact with each other, a technique of rotating one or both of the developing roller and the photoreceptor drum for a prescribed time period at a prescribed time interval in a standby state, to remove the strain.
The method disclosed in Japanese Laid-Open Patent Publication No. 02-248967, simply rotate the developing roller or the like for a prescribed time period in the standby state to eliminate the strain. However if, for example, the developer is not adhered on the developing roller, such rotation may possibly result in abnormal friction with a cleaning blade provided on the developing roller. Such friction may cause irregular wear of the member such as the developing roller.